1. Field of the Invention
The invention relates to foamable compositions, and, in particular, to compositions that are foamable in situ, for example in elongated hollow cavities such as the interior of car pillars, to produce a foam that adheres to the pillar's walls and provides a barrier to sound and/or moisture.
2. Description of the Related Art
One or more patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
In the fabrication of automobiles, trucks and similar over-the-road vehicles, many body components have cavities that require sealing to prevent corrosion of the body parts due to ingress of moisture and contaminants. This is especially true with respect to unibody structures in which a heavy solid frame is replaced with a space frame that is hollow or partially hollow. Cavities in vehicle frames also serve as passages that transmit sounds during normal use of the vehicle. For example, the upright post structure of a vehicle's body that defines a portion of the door opening presents an elongated passage or cavity that can collect moisture and contaminants and also transmit annoying sounds unless the passage or cavity is at least partially filled with a sealant material that blocks entrance of moisture and debris, and that also serves as a baffle for muting sounds. There are other articles, for example, marine vehicles, stationary machinery and buildings, that have cavities that may preferably be sealed for similar reasons or by similar means.
Many attempts have been made to seal these cavities, including spraying sealant into the cavity, introducing foam products or fiberglass matting into the cavity, and the like. These efforts have not been entirely satisfactory because of the inefficiency of the sealing and baffling methods, unsatisfactory performance of the sealing and baffling materials, or the relatively high cost of the process and materials.
Foaming in place has not been totally satisfactory because of the difficulty in controlling the placement of the foam or its precursor. In addition, an excess of the foam or its precursor is usually introduced into the body cavity. This strategy ensures that the cavity is filled and sealed; however, it also introduces an economic inefficiency. Furthermore, many foams and foam precursors include crosslinkers that are reacting during the filling process. Thus, even when processing conditions are carefully controlled, there is a time limit during which the cavity can be filled before the foam becomes rigid. In addition, foams often do not adhere to the interior surface of the cavity if it is contaminated, for example with oil. In addition, some foams or foamable products are tacky and thus cannot be easily placed in the required position. Also, certain foams and precursors deform or degrade at the elevated temperature conditions to which a vehicle body may be subjected during fabrication, which may lead to a reduction in sealing and sound attenuating properties.
Nevertheless, foaming in place has generated some scientific and commercial interest. For example, European Patent No. 0 383 498 A relates to shaped, foamable parts comprising a composition of at least 65 wt % of a base polymer containing units derived from ethylene and an olefinically unsaturated methyl acrylate having a melt index (MI) of from 0.1 to 6 and containing from 10 to 40 wt % of methyl acrylate and a cross linking agent, a chemical blowing agent and a blowing agent activator so that the part is foamable and curable concurrently at a temperature between 110-190° C. and gives a closed cell foam. The preferred MI values to achieve good foamability are between 0.2 and 1. The disclosed base polymer has a broad molecular weight distribution to provide melt strength, necessary for foaming.
In addition, U.S. Pat. Nos. 5,266,133 and 5,373,027 describe a base polymer comprising an ethylene-α,β-ethylenically unsaturated carboxylic acid copolymer ionomer whose ionic bonds are believed to encourage foaming expansion in a fairly narrow temperature range. Such ionomers are significantly more expensive than ethylene/methyl acrylate copolymers, however, which is a serious drawback for the contemplated applications.
Moreover, foamable compositions typically include plasticizers such as diethylene glycol, in order to improve the processing of the foamable composition. High viscosity at a low shear rate is needed for good foamability; often, this is achieved by selecting a base polymer with a low melt index, which encourages the formation of smaller and more uniform bubbles. The low melt index, however, makes it necessary to use a plasticizer to avoid overheating during processing. Overheating may lead to premature activation of the foaming or blowing agent. Diethylene glycol, a common plasticizer, is also said to improve adhesion. When a plasticized composition is foamed in the pillar of a car, however, and the car is exposed to high temperatures, plasticizers such as diethylene glycol often sublimate onto the windshield as a greasy deposit. This effect, which is called “fogging”, is undesirable to manufacturers and consumers alike.
There is therefore a need for a foamable composition, made of economically efficient materials, that combines ease of processing with good foaming characteristics while avoiding unwanted side effects such as fogging.